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# Unit 2: Cryptography

How have humans protected their secret messages through history? What has changed today?

## Ciphers

Assess your understanding of the code breaking presented in the ancient cryptography lesson. This series of articles and exercises will prepare you for the upcoming challenge!

## Cryptography challenge 101

Ready to try your hand at real-world code breaking? This adventure contains a beginner, intermediate and super-advanced level. See how far you can go!

## Modern cryptography

A new problem emerges in the 20th century. What happens if Alice and Bob can never meet to share a key in the first place?

## Modular arithmetic

This is a system of arithmetic for integers. These lessons provide a foundation for the mathematics presented in the Modern Cryptography tutorial.

## Primality test

Why do primes make some problems fundamentally hard? To find out we need to explore primality tests in more detail.

## Randomized algorithms

Would access to coin flips speed up a primality test? How would this work?